Optical diffusers comprising solid nanoparticles dispersed in a transparent solid matrix are known in the art.
U.S. Pat. No. 6,791,259 B1, which is incorporated by reference describes a white light illumination system comprising an LED or laser diode, a light scatterer material and a phosphor or luminescent dye material. The scatterer material preferably comprises particles dispersed in a substrate. The particles that scatter light have a size between 50 and 500 nm, preferably a size between λ/3 and λ/2, where λ is the wavelength of the emission peak of the radiation source. The nanoscatterer is integrated at the level of the active element of the source, that is, it is positioned either before the phosphor or in the phosphor, in order to scatter preferably the blue component produced by the LED or laser diode, otherwise with low divergence, and to uniform it with the yellow component scattered by the phosphor, instead produced with a wide angle of divergence. The fact that the two yellow and blue components are scattered from practically coincident scatterer centers is a necessary condition to remove the “halo” phenomenon, characterized by the presence of a dominant blue color in the direction of maximum emission, and of a dominant yellow color in the peripheral area of the light cone produced by the source, that is, to uniform color distribution of the radiation at different angles.
WO 02/089175, which is incorporated by reference, describes light sources based on UV-LED emitters with reduced dispersion of UV radiation. The light sources are LEDs which emit in the UV and which are combined with UV reflectors constituted by particles dispersed in a solid material transparent to visible light. A phosphorescent material is applied to the UV source to convert UV radiation into visible light. In a particular embodiment the phosphorescent material is applied to the surface of the UV LED and a layer of scatterer material is applied to the phosphorescent layer. The aim of this illumination device structure is to reduce the amount of UV radiation not converted into visible light and does not tackle the problem of reproducing a light similar to natural light produced by the sun and the sky.
US 2008/0262117 A1, which is incorporated by reference, describes a diffused light transmitter comprising a substantially transparent resin in which nanometric size amorphous silica particles are dispersed. In one example a material having a Haze of 71-85% and a Light Transmittance of 35-40% is reported.
None of the documents above discloses an optical sky-sun diffuser, namely a diffuser capable to carry out chromatic separation substantially with the same mechanism that gives rise to chromatic separation in nature, thereby creating the correct spectral distribution characteristic of skylight and sunlight.